Magnetic compasses have been used in navigation for several hundred years. However, as is well known, the presence of any substantial quantity of metal in proximity with the magnetic compass will cause the compass to provide a false reading. In early times, magnetic compasses were used primarily for navigating on foot, on horseback or on wooden ships. In all of these instances, the proximity of metal could easily be avoided. More recently, magnetic compasses have been used in aircrafts, automobiles and metal ships where the compass necessarily must perform in proximity with large quantities of metal. To overcome these problems, alternative forms of compasses have been proposed, such as flux gate compasses and gyroscopic compasses. Unfortunately, gyroscopic compasses are subject to precession and frequently must be reset to compensate for such precession. Moreover, both of these alternative forms of compasses are considerably more expensive than magnetic compasses to manufacture, purchase and maintain.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention. However, the following U.S. patents were considered related:
______________________________________ PAT. NO. INVENTOR ISSUED ______________________________________ 4,446,729 Watson 8 May 1984 4,146,970 Edwards 3 April 1979 4,117,602 Lapeyre 3 October 1978 3,786,571 Kelman 22 January 1974 ______________________________________
The Watson patent discloses a sailing course computer that instantaneously and continuously computes a sailboat's speed towards a predetermined destination. A polarizer disk is used as a compass card through which a light source is directed. An anlayzer disk is positioned below the polarizer disk and a photocell below the analyzer disk provides a signal indicative of the amount of light passing through the two disks in response to the sailboat assuming various headings. A second signal input is provided by a knotmeter and electronic circuitry is then used to indicate the speed of the sailboat towards its destination.
The Edwards patent discloses a remote-indicating magnetic compass for use with automobiles or the like. The system consists of a dashboard-mounted readout unit that receives and decodes logic signals provided by a remotely located electro-optical shaft encoder. The encoder consists of alternate reflective and non-reflective circular sections on a circular disc, which modulate light source, as viewed by a phototransistor mounted on a housing. The phototransistor outputs produce codes at logic levels which determine compass shaft position. The readout unit receives and decodes the coded logic signals and produces a visual display of magnetic heading.
The Lapeyre patent discloses a magnetic compass in which the compass magnet is an electro-magnet. The electro-magnet is energized only when in a horizontal disposition to provide a correct heading indication without error do to tilting of the compass magnet. The compass uses a compass disk which is coded with indicia representative of compass headings. The indicia is sensed to provide an output signal representative of the sensed heading.
The Kelman patent discloses an audio compass suitable for installation in a vehicle or boat. The compass produces an audible sound that indicates a deviation of boat or vehicle from a desired course without requiring the operator to watch the compass. The compass functions by sensing a light source such as from a photocell. The light sensed is controlled by polarized transparent sheets associated with a magnetic compass, and includes circuit that is responsive to changes in the electrical characteristic of the photocells to produce audible tones of different frequencies. A first tone indication indicates a deviation to one side of a desired heading and a second tone indicates a deviation to the other side of the desired heading.